This invention relates generally to a device for compensation of chromatic dispersion in optical fiber communication systems.
With increased demand being placed on existing optical fiber facilities, lightwave communications providers are looking for ways to increase the usable bandwidth available for customers from existing fiber without installing additional fibers. Lightwave communication systems depend on optical fiber to transport the lightwave signals from one location to another in the system.
Optical fiber, both single mode and multimode, has modal and chromatic dispersion parameters which result from material and waveguide characteristics of the fiber. Chromatic dispersion causes lightwaves at one wavelength to travel at a different velocity through the optical fiber than lightwaves at another wavelength. Thus, for example, a short pulse input to one end of the fiber emerges from the far end as a broader pulse. Pulse broadening effects and, therefore, dispersion limit the rate at which information can be carried through an optical fiber.
Several solutions have been proposed to avoid or at least mitigate the effects of dispersion. These solutions include dispersion compensation techniques.
Most high-speed fiber optic communication systems today use externally modulated lasers to minimize laser xe2x80x98chirpxe2x80x99 and reduce the effects of chromatic dispersion in the fiber. Even with external modulation, there is a certain amount of xe2x80x98chirpxe2x80x99 or broadening of the laser spectrum, because any modulated signal must contain frequency xe2x80x98sidebandsxe2x80x99 which are roughly as wide as the modulation rate. Higher bit rate transmission systems consequently have broader frequency sidebands, and at the same time can tolerate less phase delay because of the shorter bit period. Next-generation high bit rate systems are consequently very sensitive to chromatic dispersion of the optical fiber and any components such as WDM""s within the system.
Chromatic dispersion of optical fiber is roughly constant over the 1550 nm communication window, and can be compensated by several techniques including dispersion compensating fiber, FBG""s, etc. However, certain wavelength filtering components such as WDM""s can have significant dispersion characteristics due to a fundamental Kramers-Kronig type relationship between transmission spectrum and dispersion characteristics. This type of dispersion characteristic typically varies substantially over the narrow WDM passband, and therefore is difficult to compensate using conventional techniques such as dispersion compensating fiber. It is one objective of the present invention to compensate for the dispersion from WDM devices, including multiplexers, demultiplexers, and interleavers.
Conventional optical communication systems are known to utilize directly modulated semiconductor lasers. In combination with chromatic dispersion characteristics of single-mode optical fiber, chirping of these lasers contributes to the spread of optical pulses and results in intersymbol interference and overall degradation in transmission. Current and xe2x80x9cnext-generationxe2x80x9d high speed systems employ transmitters which use narrow linewidth lasers and external modulators in a window or range of wavelengths about 1550 nm. These external modulators generally have a very low chirp; some designs have a zero or negatively compensating chirp. Nevertheless, transmission distance is still dispersion-limited to about 80 kilometers at transmission rates of 10 Gb/s using conventional single mode fibers.
One solution to this problem is in the use of dispersion shifted fiber which has little dispersion in the 1550 nm window. However, replacing existing fiber with dispersion shifted fiber is costly. Other approaches have been proposed such as optical pulse shaping to reduce laser chirp, using a semiconductor laser amplifier to impose a chirp on the transmitted signal that counteracts the chirping of the directly modulated semiconductor laser.
Approaches that are more consistent with the teachings of this invention attempt to reduce the intersymbol interference at or near the receiver, or intermediate the transmitter and the receiver. Essentially any medium capable of providing a sufficient dispersion opposite to that of the optical fiber can serve as an optical pulse equalizer. For example it is known to use a special optical fiber having an equal chromatic dispersion at a required operating wavelength but opposite in sign to that of the transmitting fiber. Other methods include the use of fiber Bragg gratings as disclosed in U.S. Pat. No. 5,909,295 in the name of Li et al., and disclosed by Shigematsu et al., in U.S. Pat. No. 5,701,188 assigned to Sumitomo Electric Industries, Ltd., and the use of planar lightwave circuit (PLC) delay equalizers. Unfortunately, special compensating fiber has a high insertion loss and in many applications is not a preferable choice. Fiber gratings are generally not preferred for some field applications due to their narrow bandwidth, and fixed wavelength. PLCs are also narrow band, although tunable devices; fabricating a PLC with large dispersion equalization remains to be difficult. Shigematsu et al. disclose a hybrid of both of these less preferred choices; dispersion compensating fibre with chirped Bragg gratings.
The exact amount of dispersion compensation required for a particular installed fiber link may not be known, and may vary with wavelength or environmental conditions such as temperature. Therefore, it is desirable to have a device capable of providing a tunable amount of dispersion compensation, to simplify installation and to provide real-time control of dispersion.
In a paper entitled xe2x80x9cOptical Equalization to Combat the Effects of Laser Chirp and Fiber Dispersionxe2x80x9d published in the Journal of Lightwave Technology. Vol. 8, No. 5, May 1990, Cimini L. J. et al. describe an optical equalizer capable of combating the effects of laser chirp and fiber chromatic dispersion on high-speed long-haul fiber-optic communications links at 1.55 xcexcm. Also discussed is a control scheme for adaptively positioning the equalizer response frequency. Cimini et al. describe a device having only one common input/output port at a first partially reflective mirror and a second 100% reflective mirror together forming a cavity. The control scheme described attempts to track signal wavelength by obtaining feedback from a receiver. The amplitude response of the equalizer is essentially flat with wavelength at the input/output port, and thus, the proposed control scheme is somewhat complex requiring processing of high speed data at the optical receiver. As well, the proposed control method is stated to function with RZ signals but not with NRZ signals, a more commonly used data format. Although the equalizer described by Cimini et al. appears to perform its intended basic dispersion compensating function, there exists a need for an improved method of control of the position of the equalizer frequency response, and as well, there exists a need for an equalizer that will provide a sufficient time shift over a broader range of wavelengths. U.S. Pat. No. 5,023,947 in the name of Cimini et al., further describes this device.
A Fabry-Perot etalon having one substantially fully reflective end face and a partially reflective front face is known as a Gires-Tournois (GT) etalon. In a paper entitled Multifunction optical filter with a Michelson-Gires-Turnois interferometer for wavelength-division-multiplexed network system applications, by Benjamin B. Dingle and Masayuki Izutsu published 1998, by the Optical Society of America, a device is described which is hereafter termed the MGT device.
U.S. Pat. No. 5,557,468 in the name of Ip assigned to JDS Fitel Inc, of Nepean Canada issued Sep. 17, 1996 and shows a dual GT etalon dispersion compensator. This ""468 patent states that cascading two filters having the same reflectivity on the input/output mirrors has been suggested, but does not produce optimum results with respect to increasing the wavelength region over which the equalizer operates; The Ip patent illustrates that by cascading the etalon 100 shown in FIG. 2 with another etalon having dissimilar reflectivity characteristics and being slightly offset in its center frequency response, it is possible to favourably extend the range of the output response of the filter considerably, with respect to both time delay and in operating wavelengths. An etalon equalizer 160 in having two dissimilar cascaded etalons 162 and 164 is shown in FIG. 6 of Ip. The output response for each of the etalons 162 and 164 and the output response for the cascaded equalizer 160 is shown in FIG. 7 in the Ip patent. By cascading the etalons, the operating wavelength is doubled from 5 to 10 Ghz and the time delay is increased by about 25 percent. The first stage etalon (cavity) 162 has a first mirror with a reflectivity R1=55% serving as an input/output port; the second stage etalon (cavity) 164 has a first mirror with a reflectivity R2=38%. The nominal distance xe2x80x9cdxe2x80x9d between first and second mirrors in each cavity is 2 mm. As is shown in FIG. 7, the offset of the center operating wavelength of each of the cavities is approximately 5 Ghz which corresponds to a small difference in cavity spacing (d1xe2x89xa0d2). Although Ip""s two etalons achieve their intended purpose of extending the operation wavelength range, it would be advantageous to have a device that provide a controllable constant amount of dispersion within a wavelength band of interest. That is, where tuning allowed different constant amounts of dispersion to be induced.
The MGT device as exemplified in FIG. 1 serves as a narrow band wavelength demultiplexor; this device relies on interfering a reflected E-field with an E-field reflected by a plane mirror 16. The etalon 10 used has a 99.9% reflective back reflector 12r and a front reflector 12f having a reflectivity of about 10%; hence an output signal from only the front reflector 12f is utilized.
As of late, interleaving/de-interleaving circuits are being used more widely. These specialized multiplexor/demultiplexors serve the function of interleaving channels such that two data streams, for example a first stream consisting of channel 1, 3, 5, 7, and so on, is interleaved, or multiplexed with a second stream of channels, 2, 4, 6, 8, and so on, for form single signal consisting of channels 1, 2, 3, 4, 5, 6, 7, 8, and so on. Of course the circuit can be used oppositely, to de-interleave an already interleaved signal, into plural de-interleaved streams of channels. One such interleaver circuit is described in U.S. Pat. No. 6,125,220 issued Sep. 26, 2000 in the name of Copner et al., and another is in U.S. Pat. No. 6,040,932 issued Mar. 21, 2000 in the name of Colbourne et. al. Although interleaver circuits perform a desired function, it has been discovered that some of these circuits suffer from unwanted periodic chromatic dispersion within each channel. It is this type of periodic dispersion that can be obviated or lessened by the instant invention. It should also be noted that in many instances it is not desirable to completely eliminate all chromatic dispersion; it is believed that a small amount of such dispersion can be useful in reducing non-linear effects in WDM systems; therefore, the instant invention can be used to lessen dispersion by a required amount.
Hence, it is an object of this invention to overcome some of the limitations of the prior art described above. Furthermore, it is an object of the invention to provide a passive device that will compensate for or lessen dispersion over a plurality of interspaced wavelength channels simultaneously.
It is another object of this invention to provide a dispersion compensator that will provide a certain amount of dispersion over a predetermined wavelength band.
It is another object of the invention to provide a dispersion compensator that will provide a tunable dispersion compensator that is at least tunable over a certain range of wavelengths.
It is another object of this invention to provide a device and method for providing a tunably compensating for dispersion by provide a dispersion compensation device that provides different constant amounts of dispersion over a wavelength band of interest wherein the different amounts can be controlled by tuning the device.
In accordance with the invention a dispersion compensation device is provided for compensating dispersion in an optical signal in at least a predetermined wavelength band of wavelengths, comprising:
a first GT resonator having a first FSR and a single sloped dispersion curve in the predetermined wavelength band;
a second GT resonator having a single sloped dispersion curve in the predetermined wavelength band, the slope of the dispersion curve of the second GT resonator is opposite in sign to the slope of the dispersion curve of the first GT resonator in said wavelength band, the second GT resonator being optically coupled with the first GT resonator such that light launched into the first GT resonator is directed to the second GT resonator, at least one of the first GT resonator and the second GT resonator being a tunable resonator such that the free spectral range (FSR) thereof is controllably variable; and,
a controller for controlling the FSR of each tunable resonator and for controlling the amount of dispersion within the wavelength band.
In accordance with the invention there is further provided a dispersion compensation device comprising two etalons and a controller. At least one of the etalons is a multi-cavity etalon, and at least one of the etalons being tunable such that its FSR can be controllably varied. The controllers are for controlling the optical path length of the tunable etalons. The device is tunable so as to provide more or less dispersion over an optical channel, by varying the optical path length of at least one of the tunable etalons.
In accordance with the invention there is provided a dispersion compensation device for compensating dispersion in an optical signal, comprising:
a first optical filter having a monotonically increasing or decreasing sloped dispersion output response to light within at least a predetermined wavelength band; and
a second optical filter having a monotonically oppositely sloped dispersion output response to light within a same predetermined wavelength band, the second optical filter being optically coupled with the first optical such that light launched into the first filter is directed to the second filter, at least one of the first optical filter and the second optical filter being a tunable filter to vary the dispersion thereof over the predetermined wavelength band, such that the dispersion of the device can be controllably varied.
In accordance with another aspect of the invention a method for compensating dispersion in an optical signal is provided, comprising the steps of:
providing a first optical filter having a monotonically sloped dispersion output response within at least a predetermined wavelength band;
providing a second optical filter having a monotonically sloped dispersion output response within at least a predetermined wavelength band, wherein the first and second filters have slopes of opposite sign;
tuning the first filter in a controllable manner to vary the amount of dispersion induced thereby within the predetermined wavelength band.